1. Field of the Invention
The instant disclosure relates to a light-emitting diode (LED) current-balance driving circuit; in particular, to a LED current-balance driving circuit utilizing a current-balance coil for achieving the goal of the current balance.
2. Description of Related Art
As liquid crystal displays (LCD) are widely utilized in various fields, the traditional backlight of the LCDs utilizes cold-cathode fluorescent lamps (CCFLs), which are gradually replaced by white LEDs in order to be more environmental friendly.
Compared with the cold-cathode fluorescent lamps, the LEDs do not contain mercury and are smaller in size, longer in life-duration, and better in color saturation.
That said, since a forward bias (Vf) for each of the LEDs may not the same, a driving voltage level for each of LED strings may be different from each other as the LEDs strings are connected in parallel. Therefore, a LED current-balance driving circuit becomes necessary.
FIG. 1 shows a circuit diagram of a traditional LED current balance driving circuit for a backlight source of a display device. The traditional LED current balance driving circuit includes a power supply 20 and a current-balance circuit. The current-balance circuit includes a plurality of switching units Q1, a plurality of resistors R1, and a control circuit 30. The power supply 20 provides the driving current for driving each of the LED strings 10 according to a feedback signal (not shown in FIG. 1). The current-balance circuit is for balancing the current flowing through each of the LED strings 10. As shown in the FIG. 1, a low voltage terminal of each of the LED strings 10 is serially connected to the corresponding switching unit Q1 and the resistor R1. The control circuit 30 detects a voltage level at a high voltage terminal of each of the resistors R1, and controls operations of the corresponding switching unit Q1 according to the detected voltage level at the high voltage terminal, for adjusting the currents flowing through each of the LED strings 10 in order to ensure the currents flowing through each of the LED strings 10 can be the same.
The traditional LED current balance driving circuit requires multiple switching units and multiple resistors to achieve the goal of the current balance, which is complicated and not cost-efficient. Additionally, the power supply 20 for the traditional LED current balance driving circuit powers the backlight source by using DC to DC conversion. Due to the boost limitation associated with the DC to DC conversion, the backlight source may not be properly powered especially in the application of the display device that is larger in size, and therefore the performance of the brightness of the display device remains to be desired.
Thus, how to provide a low cost LED driving circuit for driving multiple LED strings (with the LEDs connected serially or in parallel connection), improving the performance of the brightness of the display device with large size, and balancing the currents flowing through the LED strings, is among objectives of the instant disclosure.